1. Field of the Invention
This invention relates to a method of forming self-aligned V-grooves and planar waveguides, particularly on a silicon substrate.
2. Related Art
Planar silica waveguides comprise three oxide layers (a buffer layer, a core layer and an overlay) formed on a silicon substrate. Typically, the buffer layer is 6 to 10 .mu.m thick, and can be thermally grown in steam or deposited using plasma enhanced chemical vapour deposition (PECVD). In this case, the core layer is a doped film of higher refractive index such as a 2 .mu.m thick arsenosilicate glass (AsG), or a 3 to 6 .mu.m flame hydrolysis layer (FHD) doped with germanium or phosphorus. The overlay, which is deposited after the core layer has been etched and subjected to a reflow/anneal step to remove surface roughness, is usually a 4 to 10 .mu.m thick PECVD oxide film.
V-grooves are formed in (100) silicon wafers by placing the wafers (suitably masked) in ethylene diamine pyrocatechol and water (EDP) or other anisotropic etchant. The etch rate of silicon is orientation dependent, and the slowest etching crystal planes form the side and end walls of the V-grooves. The groove sizes are controlled by the corresponding mask windows, the accuracy of their alignment with the crystal axes, the erosion rate of the edges of the mask, and the relative etch rates of the crystal planes. Typically, the finished widths of V-grooves can be controlled to within 0.5 to 1 .mu.m, such control being achieved using silicon nitride masks and EDP as the etchant.
A one dimensional array of parallel planar waveguides often needs accurate alignment with a one dimensional array of other optical components (such as parallel optical fibres or another array of parallel planar waveguides). Alignment is usually accomplished by providing both arrays with alignment means constituted by pairs of V-grooves. Several factors affect the accuracy of alignment of V-grooves and planar waveguides. In particular, the V-grooves must be etched towards the end of the processing schedule, because of the difficulty of exposing patterns on grooved wafers. Unfortunately, because V-groove etchants attack silica to varying degrees, the late etching of V-grooves will roughen exposed waveguide cores, which will adversely affect their waveguiding properties. These conflicting factors indicate that V-grooves cannot simply be formed as an adjunct to the known waveguide formation process.